Currently when the calendar is shown and the panel is set to either always autohide or intelligently autohide, the panel is hidden.
Created attachment 5807 [details] Fix panel hiding when calendar is open The attached patch fixes the issue and doesn't block the panel's autohiding when the properties dialog is shown anymore (which didn't work anymore anyway, because an old panel API call was used), because no other panel plugins do that and it doesn't seem to make much sense.
I can test this at uni next week, if you want more testing. Wouldn't there be similar issues with tasklist (in group mode), window button, clock, whiskermenu, mixer? Do they also need to call block_autohide? Thanks.
Some plugins already handle that case properly (how do you think I found out how to fix this? ;)), but to be honest I haven't tested all. So feel free and test around, the internal panel plugins *should* be fine though (might not have tested them with all options though, since you mentioned grouping...).